The invention relates to the production of semiconductor devices and of hybrid circuits. Semiconductor devices have attained in recent years a very important role in electronics and especially in computer technology. The value of such products is of the order of billions of US-$ per year. One of the repeat step during semiconductor wafer and hybrid processing is the application of a photoresist coating, which has to be removed at the end of each process step sequence. Such wafers are of the order of size of about 2.5 cm to about 20 cm diameter or more. The essentially complete stripping of the photoresist at the end of each production step constitutes a serious problem and is presently effected mainly by one of two alternate processes: one is a wet chemical process, the other is based on the use of plasma. The drawback of existing processes is that they do not warrant a complete removal of the photoresist layer unless quite extreme conditions are resorted to; application of too drastic process conditions may cause damage to the wafer-substrate and/or to the components on the wafer.
During the production process of such wafers, there is applied at each processing step a photoresist coatings. In all from 1 to 15 or even more such steps are required, each of which comprises a number of operations, from application of liquid photoresist to its stripping. The present invention is intended to overcome the drawbacks of the existing removal processes based on chemical solutions or on the use of plasma stripping where residual photoresist contamination frequently remains on some parts of the area of the wafer, and where damage occurs sometimes during plasma use, both of which, result in a certain rate of rejection and which increase production costs per device.
In the production process of semiconductors it is necessary to distinguish between the illumination and development steps which are needed for copying the mask pattern to the applied photoresist layer on the wafer and the step of complete photoresist removal--photoresist stripping--which is required at the end of each process step sequence.
Following is a short description of the common steps: After applying a layer of photoresist to the wafer the photoresist is dried by relatively low temperature which is called Soft Bake, Illumination through mask and developing are following to create (or copy) the mask pattern on the photoresist layer and only selective portions of the photoresist are removed. It is possible to use different light sources for this step like incandescent lamps, UV light sources or lasers, it is also possible to direct writing the pattern by moving one of the mentioned light sources according to the needed pattern. In this step (of creating or copying the mask pattern) the energy of the light source is low as the photoresist is soft baked after low temperature drying and still light sensitive. The process which is described in U.S. Pat. No. 4414059 by S. E. Blum is related to the photoresist removal during a given step and not to the stripping at the end of such process step which requires different process parameters.
Following the pattern creation step the photoresist passes an extra drying process called "Post Bake" or "Hard Bake" which is done at a higher temperature. After this step the photoresist is more stable and ready for the next steps like etching or ion implantation which further hardens the photoresist. At the end of each of such sequence of operations, it is necessary to completely remove the photoresist for the next process step. The present invention relates to the complete stripping of Hard Baked photoresist at the end of each process cycle and at the end of the last processing step.
The process of the invention is one of ablative stepwise removal (stripping) of the photoresist by a sweep of a beam of high energy pulses in the UV wave length range , each of which pulses removes a certain depth of the photoresist layer.